Thin wall, solid-wood box construction is designed to flex and sing along with the drivers, in a controlled way, rather than try to fight the back wave with heavy highly damped box construction like other speakers. Polypropylene-like material used to make the midrange driver cones have controlled flex, so they sound a little softer and smoother than rigid cones or paper.
Harbeth is considered an easy to listen to speaker, sacrificing a little detail to avoid harshness. Warm and safe sound, typical of the British hifi sound. The Compact 7 and the M40 are the best of the lineup, imo.